Methods for filling superconductive magnets are disclosed. An apparatus and method for reducing quenches when filling a superconductive magnet with liquid helium from a cryostat (dewar). The method will reduce helium losses which can occur during the transfer of liquid helium from the dewar to the superconductive magnet.
Magnetic resonance imaging (MRI) and nuclear magnetic resonance (NMR) systems employing superconductive or other types of magnets are employed in fields such as medical diagnostics. The superconductive magnets comprise a coil assembly having a main coil which is at least partially immersed in liquid helium contained in a helium reservoir. The reservoir is typically surrounded by dual thermal shields which in turn are surrounded by a vacuum enclosure. Nb—Ti superconductive coils typically operate at a temperature of approximately 4 Kelvin, and Nb—Sn superconductive coils typically operate at a temperature of approximately 10 Kelvin. When the coil assembly is cooled to such a temperature it becomes superconductive and the magnet field strength is maintained without significant further energy input.
A necessity for the operation of a superconductive magnet is the presence of a coolant. This coolant can be liquid helium which can achieve the low temperatures necessary to allow the material of the magnet coil to reach a superconductive state. This need for low temperatures necessitates that the reservoir in the magnet must be filled with a sufficient amount of liquid helium at a cold enough temperature to allow the magnet coils to become superconducting. The magnet must be filled with the liquid coolant before the superconducting coils can be energized.
The common practice in filling these magnets is to transfer a push gas into a dewar containing liquid helium until a whistling sound is heard from the dewar. This whistling sound indicates that the cold gaseous helium is now entering the dip tube and that no more liquid helium can be withdrawn from the dewar. When the whistling sound is heard, the transfer of the liquid helium is immediately stopped and a new full dewar is connected if the desired level of liquid helium has not been achieved with the now empty dewar.
Dewars are available with built in level meters; however, these dewars can be very expensive and their level meters often inaccurate.
The whistling sound further indicates that either cold gaseous helium or a two phase flow of cold gaseous helium and liquid helium is entering the cryostat of the magnet. This is undesirable because the cold gaseous helium may cause the superconductive magnet to quench. Further, the cold gaseous helium will not collect as a liquid in the cryostat of the superconductive magnet. The gas will be vented to the atmosphere through the vent/relief valve or by-pass valve of the magnet resulting in a complete loss of this helium from the transfer process system.
Another problem that can occur when transferring liquid helium from a dewar into a cryostat of a superconductive magnet, is slow filling. In this case the transfer rate of liquid helium is lower that the normal transfer rate, which typically is 6-9 liters per minute (lpm) transferred out of the dewar.
Slow fill is indicative of an abnormal condition in the transfer system. Such a condition could be loss of vacuum in the transfer lines, insufficient dewar pressure, formation of ice or frozen air in the transfer line or at the magnet inlet or mail function of the superconductive magnets vent/pressure relief valve.
When a slow fill is observed a set of corrective actions must immediately be taken otherwise there is an increased risk of a quench occurring and very often slow fills results in high losses of liquid helium.
Therefore it is desirable to detect a slow fill a soon as possible after the transfer process has begun.
Traditionally a slow fill is detected by observing the increase of the superconductive magnets level meter over a certain time, typically 5 to 10 minutes, however a disadvantage of this method is that the reliability of the observations depends on the accuracy of the superconductive magnets level meter and the total capacity of the cryostat which varies from magnet type to magnet type. This imprecision can lead to errors in diagnosing the slow fill and can result in quench and loss of helium.